According to one aspect of the present invention there is provided a copper-electroplating solution containing the complex X Cu(P O X being an alkali metal, and an alkali metal pyrophosphate, together with at least 0.5 ppm. of an additive containing the grouping Where R is hydrogen or a metal.

According to another aspect of the present invention there is provided an aqueous solution from which copper can be deposited electrolytically, made by dissolving a copper salt and an alkali metal pyrophosphate in sulficient quantities to produce the complex X Cu(P O Where X is the alkali metal, and to leave an excess of alkali metal pyrophosphate, and from 0.5 p.p.m. to saturation of an additive containing a -mercapto-lc3z4-thiadiaz-Z-yl radical.

The invention stems: from the discovery that bright and smooth deposits of cop-per can be produced by electrodeposition from pyrophosphate copper-electroplating solutions containing, as additives, the mercaptothiadiazoles referred to above.

The additive is preferably of the formula:

a being a low integer.

The second and third of these compounds: Z-mercapto- S-methylmercapto-l23:4-thiadiazole and 2 -mercapto-5- n-butylmercapto-l:3z4-thiadiazole, may be prepared by the reactions between the monosodium salt of 2:5-dimercapto-lzS :4-thiadiazole and the appropriate alkyl halides, preferably the iodides, e.g., methyl iodide and nbutyl iodide respectively. The fourth compound: 2:2- (dimercaptomethane) bis (5 mercapto 1:3:4 thiadiazole) may be prepared by the reaction between the monosodium salt of 2:5-dimercapto-l:3:4-thiadiazole and methylene di-bromide, whilst a compound of the fifth formula: bis-2:2-(5-mercapto 1:3 :4 thiadiazyl) disulphide (a having the value 1), together with some other compounds of the fifth formula in which a is greater than 1, may be prepared by the reaction between 2:5-dimercapto-1 :3 :4-thiadiazole with iodine.

The concentration of copper in the solution is not critical and may vary from less than 1 gin/liter to saturation. A preferred concentration is, however, between 25 3,lhl,575 Patented Dec. 15, 1964} and 30 gmsJliter. Further, although any of the alkalimetals may be used it is preferred to use potassium or sodium. The molar ratio of pyropho'sphate to copper in the solution is necessarily greater than 2.0:1 as there is pyrophosphate in the solution additional to that in the complex X Cu(P O The ratio is preferably not greater than 2.5 :1 andis preferably not less than 2.321.

The solution may be acid or alkaline, but if acid the pyrophosphate tends to become converted into orthophosphate making the process uneconomical. Further, if the pH is greater than about 12 or 13 copper hydroxide is precipitated. The preferred rangeof pH is 7.5 to 9.5, and the best results are usually obtained with a pH of substantially 8.5.

Gther materials may be included in the solution. Ammonia, for example, may be added to promote anode dissolution during electrodeposition, and is preferably present in a concentration of between 1 and 3 gms./liter. For the same purpose there may be included an aliphatic carboxylic acid, such as citric acid, tmtaric acid or oxalic acid, or there may be included an alpha-amino acid. In addition to promoting anode dissolution, the carboxylic acids have the effect of increasing the range of current density which may be used to obtain electrodeposits of where R is hydrogen or a metal, such materials being soluble in water to produce a solution in accordance with either of the aspects of the invention hereinbefore set forth.

The solution may conveniently be prepared from the material by dissolving the alkali metal pyrophosphate, preferably in the form of anhydrous tetrasodium or tetrapotassium pyrophosphate, in water and then adding the copper salt, preferably copper pyrophosphate. If ammonia is to be included this is added next and is followed by the additive, which preferably has been dissolved in a solution of potassium hydroxide. The pH of the solution may be adjusted by adding pyrophosphoric acid, tetra phosphoric acid or nitric acid, or by adding sodium hydroxide, potassium hydroxide or ammonia solution.

From yet another aspect the invention consists in a method of coating a metallic article with copper comprising immersing the article in a solution in accordance with either of the first two aspects of the invention hereinbefore set forth, and passing an electric current through the solution with the article as acathode.

Copper may be deposited on any of the usual basis metals such as iron, steel, copper and copper alloys, nickel and lead; further, when the alkalinity of the solution is relatively low, as is that of the preferred solution, copper may be deposited on light metals such as aluminium and zinc.

Deposits of copper for decorative purposes would normally be up to about 0.002 inch thick, but much thicker deposits may be produced, as for electroforming purposes, which may be from 0.002 inch to one inch thick or even thicker. Little or no mechanical after-treatment of the copper is normally needed after deposition for electro forming.

The temperature of the solution is preferably between 95 and 140 F. and the most satisfactory range of temperature has been found to be between 130 and 140 F.

The current density is not critical but the appearance of the deposit is found to vary with the current density employed. Below a current density of about amps./ sq. ft. the deposit is usually light pink in colour, and above a current density of about 60 amps/sq. ft. the deposit is often found to be dull and may be powdery and nonadherent. In general the higher is the copper content of the solution, the higher is the maximum current density which can be employed. Current densities of up to 80 amps/sq. ft. have been successfully employed, but when using a cathode of irregular shape it has been found virtually impossible to achieve satisfactory deposition using a current density of above 60 amps./ sq. ft.

the pH of the solution was 8.9, the current density was 60 amps/sq. ft. nominal and the temperature was 130 F. A deposit of copper 0.0003 inch thick was obtained after 5 minutes plating time. The deposit was bright and well levelled, and was suitable for nickel plating without any mechanical polishing.

In a similar experiment, under the same conditions except that the time of plating was extended to 8.5 minutes, the deposit was 0.0005 inch thick.

(2) After preliminary cleaning a metal article was plated in a solution of copper pyrophosphate'and tetrapotassium pyrophosphate containing Copper gms./liter 30 Pyrophosphate do 200 Ammonia (NH do 2 Bis-2:2-(5-mercapto-1 :3 :4-thiadiazyl) disulphide p.p.m-- 5 The pH of the solution was 8.7 and the temperature maintained between 130 and 140 F. The current density was 40 amp./ sq. ft. and after 2 hours a bright smooth deposit of copper was formed approximately 0.004 inch thick.

Similar results were obtained when the additive was replaced with the following additives: (a) Z-mercapto-5-methylmercapto-1 :3 :4-thiadiazole (b) 2-mercapto-5-n-butylmereapto-l 3 :4-thiadiazole (c) 2:2-(dimercaptomethane) -bis-(5-mercapto-1:3:4-

thiadiazole) We claim:

1. The method of electroplating a bright, well-levelled deposit of copper upon a metal substrate which comprises:

(a) providing an aqueous'solution having a pH between about 7 .5 to 9.5 containing dissolved therein:

(1) a copper salt complex of the formula where X is alkali metal,

(2) alkali metal pyrophosphate, and

(3) from 0.5 p.p.m. to saturation of an additive having the formula:

wherein R is selected from the group consisting of hydrogen and metal atoms, and R is selected from the group consisting of hydrogen, metal atoms, alkyl radicals of 1 to 4 carbon atoms, and radicals of the formula:

TF2 -CH s- 2- \S/ S R (b) maintaining the temperature of said solution at between about to F.,

(c) placing a metal substrate in said solution, and

(d) passing a DC. electric current of a current density between about 5 to 60 amps./ sq. ft. through the solution between said metal substrate as a cathode and a copper anode until a copper deposit of the desired thickness is formed upon the metal substrate.

2. In the method of electrodepositing copper on metal substrates from an alkaline copper pyrophosphate electroplating bath, the improvement which comprises including in the bath as an additive to improve leveling and brightness of the deposited copper layer from 0.5 p.p.m. to saturation of a heterocyclic organic compound selected from the group consisting of:

3. The method of electroplating a bright, well-levelled deposit of copper upon a metal substrate which comprises:

(a) providing an aqueous solution having a pH between about 7.5 to 9.5 containing dissolved therein:

(1) a copper salt,

(2) an alkali metal pyrophosphate in an amount at least sufiicient to form with said copper salt the complex X Cu(P 0 where X is alkali metal, and

(3) an additive containing the moiety Hi L wherein R is selected from the group consisting of hydrogen and metal atoms, (b) maintaining the temperature of said solution at between about 95 to 140 F., (c) placing a metal substrate in said solution, and (d) passing a DC. electric current of a current density between about 5 to 60 amps./ sq. ft. through the solution between said metal substrate as a cathode and a copper anode until a copper deposit of the desired thickness is formed upon the metal substrate.

4. A copper-electroplating solution from which a bright, well-levelled deposit of copper may be electrodeposited upon a metal substrate suitable for nickel plating without mechanical polishing, said solution consisting essentially. of the following ingredients dissolved in water in the amounts as specified:

(a) a copper salt (b) an alkali metal pyrophosphate in an amount at least sufiicient to form with said copper salt the complex X Cu(P O where X is alkali metal, and (c) an additive containing the moiety wherein R is selected from the group consisting of hydrogen and metal atoms.

6. An aqueous solution for electroplating a metal substrate with copper consisting essentially of the following ingredients dissolved in water in the amounts specified:

8. An aqueous solution for electroplating a metal substrate with copper consisting essentially of copper pyrophosphate, tetrapotassium pyrophosphate, ammonia and bis-2,2'-(5-mercapto-1,3,4-thiadiazyl) disulfide dissolved in water to provide the following concentrations of solution ingredients:

9. An aqueous solution as claimed in claim 5 which also contains at least one further additive selected from' (c) from 0.5 part per million to saturation of an additive of theformula:

wherein R is selected from the group consisting of hydrogen and metal atoms and R is selected firom the group consisting of hydrogen and metal atoms, alkyl radicals having from 1 to 4 carbon atoms, and radicals of the formula:

ll IN CH S (J3 i l-SR 11. An aqueous solution as claimed in claim 5 wherein said additive is present in an amount of at least 0.5 ppm.

12. A water-soluble solid composition which comprises:

(a) a water-soluble copper salt,

(b) an alkali metal pyrophosphate in an amount at least sufficient to form with the said copper salt the complex X Cu(P O- where X is alkali metal, and

(c) an additive containing the moiety:

wherein R is selected from the group consisting of hydrogen and metal atoms. 13. A water-soluble solid composition as claimed in claim 12, wherein the said additive is of the formula:

il l RSC JSR' wherein R is selected from the group consisting of hydrogen and metal atoms, and

R is selected from the group consisting of hydrogen and metal atoms, alkyl radicals of 1 to 4 carbon atoms, and radicals of the formula:

N-N -on2-s-i J-SR 14. A solid composition as claimed in claim 13 wherein the said alkali metal pyrophosphate is selected from the group consisting of sodium and potassium pyrophosphates and is present in an amount sufiicient to provide a molar ratio of pyrophosphate to copper of between about 2.021 and 2.5 :1.